The present invention relates generally to laser systems and more particularly to control systems and methods for controlling the phase of optical signals output by laser systems by utilizing frequency shifting techniques.
Lasers are presently employed for a wide variety of applications. For example, lasers are employed to process materials, such as by cutting, welding, heat treating, drilling, trimming and coating materials, stripping paint, removing coatings, cleaning surfaces, and providing laser markings. Lasers are also used in many medical applications for precision surgery. Additionally, lasers are used in military applications, including laser weapon and laser ranging systems. Laser communication systems have also been developed in which laser signals are transmitted in a predetermined format to transmit data.
Along with the ever increasing number of applications in which lasers are used, the demands on the laser systems are also ever increasing. For example, a number of applications, including military, materials processing, medical, and communications applications, demand continuous wave lasers which emit increasingly higher power levels. In addition, a number of applications demand that the laser system produce an output beam which is of high quality, such as by being diffraction limited and/or exhibiting predominantly or entirely fundamental or TEM00 mode characteristics. Accordingly, the output beam can be more definitely focused to achieve higher brightness. At the same time, many applications require that the laser system produce an output beam which is adaptable or dynamically controllable.
One example of the need for high power, high quality laser beams is illustrated by the laser devices used to focus on remote targets. In these applications, it is advantageous for the laser beam to achieve a maximum brightness at the location of the target. For example, in military applications, it is advantageous to generate a laser beam that is focused on the remote target with maximum intensity. Similarly, in medical applications, it is essential that the laser beam be focused on the target tissue such that surrounding tissue is not affected.
Several different types of laser devices that generate high power laser beams have been developed by The Boeing Company, assignee of the present application. Examples of these laser device are discussed in detail in U.S. Pat. No. 5,694,408 to Bott et al. and U.S. Pat. No. 5,832,006 to Rice et al., the contents of which are incorporated herein by reference.
The basic approach of these laser devices is to amplify a coherent signal emitted from a master oscillator using a phased array of fiber optic amplifiers. A sample of the output optical signal is extracted for comparison to a reference laser beam that has also typically been output by the master oscillator. The sample of the output optical signal and the reference signal are combined by interference, and the interference signal is sampled by an array of detectors. The difference in phase between the sample of the output optical signal and the reference signal is recorded by the detector, and is used as feedback for altering the phase modulation of the output optical signal via an array of phase modulators that are in optical communication with respective fiber optic amplifiers.
In one example, it may be desired that the plurality of output optical signals be capable of being combined into a diffraction limited signal, thereby requiring that the output optical signals emitted by the fiber optic amplifiers have a constant phase front. Unfortunately, optical path disturbances including those attributable to variations in the optical path length may differently affect the elements of the phased array, thereby requiring independent modulation of the optical signals propagating through the respective fiber optic amplifiers in a manner that is capable of being varied over time as the optical path disturbances vary. These optical path disturbances may be due, for example, to platform shock and vibration, turn-on transients and pumping power fluctuations. Moreover, these optical path disturbances may be many wavelengths and, in some instances, thousands of wavelengths in magnitude and may occur very quickly so that a wide control bandwidth is required.
While a flat phase front is often desirable, some applications will require other types of phase fronts. For example, in one application, a reference beam is initially transmitted to a target of interest. By monitoring the reflection of the beam, atmospheric turbulence in the path of the output laser beam may be detected. To counteract this turbulence, the laser device may desirably alter the phase of the signals emitted by the various fiber optic amplifiers such that the output laser beam has a wavefront that compensates for the atmospheric turbulence.
To provide the desired phase front, the laser devices described by U.S. Pat. Nos. 5,694,408 and 5,832,006 have a feedback loop and an array of phase modulators that control the phase modulation of the output laser beam. Specifically, as discussed, a portion of the output laser beam is combined through interference with a reference signal to determine the phase difference for the signals emitted by each fiber optic amplifier. By use of the feedback signal representative of the phase of the output laser beam and knowledge of the desired wavefront, the output laser beam can be generally controlled via the array of phase modulators to produce the desired wavefront and/or to appropriately steer or tilt the wavefront.
Although these laser systems, for the most part, provide reliable and accurate control of the output laser beam, U.S. Pat. No. 6,233,085 to Bartley C. Johnson, the contents of which are also incorporated by reference herein, describes the feedback loop and the associated array of phase modulators in more detail. In this regard, the control methodology described by U.S. Pat. No. 6,233,085 patent can provide for a wide range of phase modulation by avoiding saturation and uncontrolled modulation changes in the output signal.
Although generally effective, the control methodologies described heretofore in conjunction with fiber optic phased arrays have typically been implemented utilizing digital signal processing techniques. In some applications, such as those applications potentially involving military combat, the reliability of digital signal processing techniques is still questioned. As such, it would be desirable to develop a control system and method for providing wide band phase control for the output optical signals of a fiber optic phased array utilizing conventional analog electronics.
A fiber optic phased array, as well as associated methods and apparatus for controllably adjusting the frequency of the optical signals emitted by a fiber optic phased array, are provided that permit wide band phase control and may be implemented, if desired, utilizing conventional analog electronics. In this regard, the method and apparatus of the present invention can independently control the phase of the optical signals propagating through each fiber optic amplifier of a fiber optic phased array, even in instances in which optical phase disturbances that are many wavelengths, and perhaps thousands of wavelengths, in magnitude occur, such as due to platform shock and vibration, turn-on transients, pumping power fluctuations and the like. As such, the control method and apparatus of the present invention permit a fiber optic phased array to generate a flat phase front that, in turn, can provide a diffraction limited output laser beam. Alternatively, the control method and apparatus may be designed such that the output signals emitted by an array of fiber optic amplifiers has any other desired phase front, such as to compensate for atmospheric perturbations or the like. Since the control method and apparatus of the present invention are capable of being implemented by conventional analog electronics, the control method and apparatus of the present invention may also be more readily adopted for use in demanding applications, such as combat applications in which the reliability of sophisticated digital electronics may well be questioned.
According to one aspect of the present invention, an apparatus for controllably adjusting the frequency of an optical signal is provided. The apparatus includes a detector assembly for receiving an interference signal generated by optical interference of the optical signal and a first reference signal. The detector assembly then generates a detected output signal having a frequency equal to the difference between the respective frequencies of the optical signal and the first reference signal. In one embodiment, the detector assembly is an optical heterodyne receiver and may include, for example, a photodetector.
The apparatus of this aspect of the present invention also includes a mixer, such as a double balanced mixer, for mixing the detected output signal generated by the detector assembly and a second reference signal. The mixer generates a mixed output signal having a frequency equal to the difference between the respective frequency of the detected output signal and the second reference signal. The apparatus of this aspect of the present invention also includes a voltage-controlled oscillator for generating a feedback signal in response to the mixed output signal. For example, the voltage-controlled oscillator may be a radio frequency (RF) voltage-controlled oscillator having a frequency control port adapted to receive the mixed output signal. The apparatus of this aspect of the present invention further includes a frequency translator for adjusting the frequency of the optical signal in response to the feedback signal. For example, the frequency translator may be an acousto-optic frequency translator for adjusting the frequency of the optical signal in response to the frequency of the feedback signal.
As such, the voltage-controlled oscillator generates the feedback signal to have a frequency that causes the frequency translator to adjust the frequency of the optical signal so as to cause the frequency of the mixed optical signal to be reduced. In one advantageous embodiment, for example, the voltage-controlled oscillator generates the feedback signal to have a frequency that causes the frequency translator to adjust the frequency of the optical signal so as to drive the frequency of the mixed output signal toward zero. By adjusting the frequency of the optical signal in the manner described in accordance with this aspect of the present invention, the phase of the optical signal may be effectively matched with the phase of the first reference signal even as substantial optical path disturbances arise.
Although described in conjunction with an apparatus, a method for controllably adjusting the frequency of an optical signal is also provided in accordance with another aspect of the present invention. In this regard, a detected output signal is generated that has a frequency equal to the difference between respective frequencies of the optical signal and a first reference signal. In one embodiment, for example, the detected output signal is generated based upon an interference signal created by the optical interference of the optical signal and the first reference signal. Thereafter, the detected output signal is mixed with a second reference signal so as to generate a mixed output signal having a frequency equal to the difference between the respective frequencies of the detected output signal and the second reference signal. A feedback signal is then generated in response to the mixed output signal. The frequency of the optical signal is then adjusted in response to the feedback signal. In this regard, the feedback signal may be generated so as to have a frequency that causes the frequency of the optical signal to be adjusted so as to reduce the frequency of the mixed output signal, such as by driving the frequency of the mixed output signal toward zero.
The control method and apparatus of the present invention may be utilized in conjunction with a fiber optic phased array in accordance with another aspect of the present invention. In this regard, the fiber optic phased array includes a plurality of fiber optic amplifiers for individually amplifying respective optical signals. The fiber optic phased array also includes a plurality of frequency translators, such as a plurality of acousto-optic frequency translators, associated with respective fiber optic amplifiers for adjusting the frequency of the respective optical signals. For example, the plurality of frequency translators may adjust the frequency of the optical signals prior to amplification by the respective fiber optic amplifiers.
The fiber optic phased array also includes a detector assembly for receiving a plurality of interference signals associated with respective ones of the fiber optic amplifiers. Each interference signal is generated by optical interference of the optical signal emitted by a respective fiber optic amplifier and a first reference signal. In one embodiment, the detector assembly may include a plurality of optical heterodyne receivers, including a plurality of photodetectors, for receiving respective interference signals. Each optical heterodyne receiver may generate a detected output signal having a frequency equal to the difference between the frequency of the optical signals emitted by the respective fiber optic amplifier and the frequency of the first reference signal.
The fiber optic phased array of this aspect of the present invention also includes a feedback assembly for directing the frequency translators to adjust the frequency of the optical signals propagating through the respective fiber optic amplifiers so as to maintain a predefined frequency relationship with respect to the first reference signal. The feedback assembly may include a plurality of mixers, such as a plurality of double balanced mixers, for mixing the detected output signal generated by respective optical heterodyne receivers and a second reference signal. In this embodiment, the plurality of mixers generate respective mixed output signals having a frequency equal to the difference between the frequency of the respective detected output signal and the second reference signal.
The feedback assembly may also include a plurality of voltage-controlled oscillators for generating feedback signals in response to the respective mixed output signals. The feedback signals direct the respective frequency translators to adjust the frequency of the optical signals propagating through the respective fiber optic amplifiers so as to maintain the predefined frequency relationship with respect to the first reference signal. In one embodiment, for example, the feedback assembly may direct the respective frequency translators to adjust the frequency of the optical signals propagating along the respective fiber optic amplifiers so as to maintain a frequency difference with respect to the first reference signal equal to the frequency of the second reference signal.
The fiber optic phased array may include a variety of other components. For example, the fiber optic phased array may include a beam splitter for splitting the optical signals emitted by the fiber optic amplifiers such that one portion of the optical signals emitted by each fiber optic amplifier is output and another portion of the optical signals emitted by each fiber optic amplifier is directed to the detector assembly. The fiber optic phased array may also include a master oscillator for providing the optical signals to the plurality of fiber optic amplifiers.
By utilizing at least aspects of the control method and apparatus of the present invention, the fiber optic phased array can provide a desired phase front by appropriately frequency shifting the optical signals propagating through the respective fiber optic amplifiers. In this regard, the predefined phase front may be a flat phase front so as to permit a diffraction limited output laser beam. Alternatively, the phase front may have a predefined shape, such as to compensate for atmospheric perturbations or the like. Moreover, the fiber optic phased array, including aspects of the control method and apparatus of the present invention, is capable of providing the desired phase front, even if there are substantial optical path disturbances on the order of many wavelengths, such as thousands of wavelengths, in magnitude. Moreover, the detector and feedback assemblies of the fiber optic phased array of this aspect of the present invention may be implemented utilizing conventional analog electronics and therefore be more readily adopted in at least some applications.